Mechanisms of Protein Stabilization During Freeze-Drying Storage: The Relative Importance of Thermodynamic Stabilization and Glassy State Relaxation Dynamics

INTRODUCTION Partly because of chemical complexity and partly due to the marginal stability of higher order structure (i.e., conformation), therapeutic proteins often present significant stability problems. While proteins are generally quite stable in aqueous solution for short periods of time, a pharmaceutical product must have adequate stability over storage periods of many months, typically several years. Many proteins do not possess this long-term stability in the aqueous state. Ironically, while the nature of water is an important contributing factor to the conformational stability of a protein, water is a destabilizing factor in the longterm preservation of the chemical and structural integrity of a protein. With proteins, as with most labile molecules, removal of water to form a solid generally improves storage stability. Thus, proteins are typically freeze-dried in an attempt to achieve adequate storage stability. However, some proteins suffer irreversible damage during freeze-drying, and even if the protein survives freezedrying without damage, the freeze-dried solid does not always have the desired storage stability. Stability problems are normally minimized by a combination of proper process control and formulation optimization.